International Application No. WO 9503649 describes a two-component current controller with a space vector modulator for an induction machine supplied by a pulse-controlled inverter. This field-oriented control system includes an input transformer system, an EMF computer, a pre-control network, a current control circuit, an active current controller, and an output-side coordinate converter. The input-side transformer is supplied with two measured phase currents from which orthogonal field-oriented actual current components are generated using a flux angle. These actual current components are sent both to the current control circuit and to the EMF computer. The EMF computer also receives the setpoint voltage components, the parameters of the induction machine (stator resistance R.sub.s and leakage inductance L.sub..sigma.), and the stator frequency .omega..sub.s. The output of the EMF computer is coupled with the actual-value input of the active current controller, so that a field-oriented, torque-forming setpoint current component appears at the setpoint-value input of this control system. The output signal of this active current control system is used as a speed-correction for a calculated slip frequency, which is added to a measured actual rpm value to form the stator frequency. The current-control circuit contains a comparator with a downstream controller for each field-oriented current component, so that an output signal of the input-side transformer exists at each inverting input, and a field-oriented setpoint current component exists at each non-inverting input. The outputs of this current-control circuit are connected to the outputs of the pre-control network; the setpoint current values, the "stator resistance," "leakage inductance," and "magnetizing inductance" parameters are fed to the input of the pre-control network. The sum signal, consisting of field-oriented pre-control values and controller manipulated variables, also known as field-oriented manipulated variables, is fed to the output-side coordinate converter, which converts these orthogonal components into polar components. These polar manipulated variable components, also called voltage components, and the stator frequency are fed to the space vector modulator, at whose outputs control signals for the pulse-controlled inverter are formed.
For this field-oriented control system with a sensor, active current control is used to correct the slip frequency and to adjust parameters (rotor resistance). The slip frequency is calculated from the torque-forming setpoint current component and a quotient of the rotor resistance and setpoint flux. These two signals are multiplied, and the product is equal to the slip frequency. Since the "rotor resistance" parameter is temperature-dependent, the slip frequency changes in proportion to the "rotor resistance" parameter. Using this active current control method, the correct slip frequency may be determined, allowing maximum torque to be developed.
This control structure has also proved effective for a sensorless induction machine. In a field-oriented controller system without a sensor, an EMF control system is used instead of active current control. The actual input value of the EMF system is coupled with a d-component output of an EMF computer of the field-oriented control system. The integral portion of the PI controller for this EMF control system represents the rotational speed, including the slip frequency correction. The absolute value of slip frequency correction is practically negligible at lower speeds. In this manner, the EMF control system delivers a good estimated value for the speed at medium and higher speeds.
The EMF computer, which computes an EMF actual space vector as a function of the actual current component, the setpoint voltage component, and the machine parameters uses the so-called "voltage model." Since this voltage model is very inaccurate at low frequencies because of the low absolute value of the motor voltage, field-oriented operation at speeds approaching zero is not easily accomplished. At zero frequency, this process is no longer useable. In particular, reversing without undesired changes in torque is very difficult. Also, strategies for starting up from rest, and for decelerating to rest, must be found.
There are various conventional methods of implementing sensorless operation of asynchronous machines. At medium and higher speeds, sensorless operation using the conventional methods of field-oriented control may be used satisfactorily. There are various approaches for lower frequencies.
A method for sensorless field-orientation of rotating-field machines down to frequency zero is described in the article "Feldorientierung der geberlosen Drehfeldmaschine," (Field-orientation for sensorless rotating-field machines) which appeared in the German periodical "etz", issue 21, 1995, pages 14-23. In this method, dynamic current signals are injected into the flux-forming current component of the stator current. This injection has little dynamic influence on the torque produced. In stationary operation, the model works perfectly. The effect of this excitation is evaluated in the measured machine voltages and currents. The position of the rotor flux axis may be estimated using the saturation characteristics of the rotor leakage, which assures field-orientation. This procedure must be replaced by an appropriate field-oriented control system in the upper rotational speed ranges, since it can only be used in the saturation range of the rotor leakage, and it also requires a sufficient voltage reserve to supply the test signal. The machine used must have a distinct rotor current saturation characteristic. The implementation of this method is very expensive and computation-intensive because of the required vector transformation.
The article "High-dynamic AC machine control without speed or position sensor," ETEP, vol. 6, #1, January-February 1996, pp. 47-51, describes a method in which the voltage model of the machine is supported by the current model at low frequencies. The dynamics of this model are limited by the rotor time constant of the machine. Additionally, the current model does not produce the proper flux angle necessary for orientation, but rather only the rotor flux amplitude. Because of this, it is suitable, for this application, only as an observer of parameter compliance for the voltage model. Using this method, it is possible, in principle, to approach zero rpm and eliminate the effects of thermally-caused fluctuations in the stator resistance.